Method of quenching metal filament in froth

ABSTRACT

A process for directly producing metallic filaments and fibers from molten metal by extruding a stream of the molten metal into a froth quenchant. The froth serves to quickly solidify the molten metal stream without damaging inertial effects, thus making the process ideally suited for metals having a high latent heat of fusion and/or a high surface tension.

United States Patent 1151 3,685,568 Pond, Sr. 1 51 Aug. 22, 1972 [54]METHOD OF QUENCHING METAL 2,976,590 3/l96l Pond ..l64l82 FILAMENT INFROTH 3,2l6,076 l [/1965 Alber et al 164/82 X 3,347,959 10/1967 Engelkeet a]. ..l64/82 X [72] a?" westmmste" 3,543,831 12/1970 Schile ..l64/86x [73] Assignee: United States Steel Corporation P i Examiner R SpencerAnneal- 22 Filed: March 1 1971 Attorney-Forest C. Sexton [21] Appl. No.:119,752 57 ABS A process for directly producing metallic filaments [52]US. Cl. ..l64/89, l64l82, 264/ 176 F and fibers from mom metal byextruding a stream of [5i] I ll. Cl. 11/12 v the molten metal into afroth quenchant The froh Fleld of Search 89, Serves to the molten streamwithout damaging inertial effects, thus making the [56] References cuedprocess ideally suited for metals having a high latent UNITED STATESPATENTS heat of fusion and/or a high surface tension.

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Attorney BACKGROUND OF THE INVENTION It is well accepted that metallicfilaments and fibers can be formed by nondrawing processes involvingdirect casting techniques. Direct casting renders a wirelike productwithout distortion of the metal grain form and therefore withoutsubstantial residual stresses. Although early processes for such directcasting involved the mere casting of molten metal into grooves, morerecent advancements have resulted in improved processes which usuallyinvolve extrusion of a continuous stream of molten metal, and thensolidification of the stream in flight while it is in the extrudedfilamentary form. solidification of the extruded form may be effected bypassing the liquid stream through a cooling atmosphere, or by extrudingthe liquid stream onto a rotating or moving chill plate, the motion ofwhich is sufficient to maintain the extruded molten metal in itsfilamentary form until solidified.

In my US. Pat. No. 2,879,566, I describe another process for directcasting of metal filaments wherein a continuous stream of molten metalis extruded into contact with an unconfined gaseous jet stream. The jetstream not only serves to quench the molten metal, but further serves tosupport the molten metal in its filamentary form.

The manufacture of fine wires or filaments by the above patented processfinds a limitation when the latent heat of fusion of the metal issufficiently high and/or the surface tension of the metal sufiicientlylarge to cause the extruded metal stream to be pinched off beforesolidification can occur. The result is that these metals, such as iron,steel, copper, nickel, beryllium and boron, for example, can beprocessed to produce only short or extremely short fibers. In order toproduce a filament of appreciable length from these metals, it isapparent that solidification of the metal stream must be effected morequickly and closer to the extrusion nozzle than is possible in air orother gaseous coolants at ambient temperatures. Although water or otherliquid quenchants would indeed solidify a metal filament very quickly asnecessary, liquid quenchants are not suitable because of the excessiveinertial effect the liquid would have on the metal filaments. That is tosay, a rapidly moving molten metal stream disintegrates when it strikesa massive liquid surface.

SUMMARY OF THE INVENTION This invention is predicated upon mydevelopment of a new method for producing metallic filaments and fibersutilizing a froth quenchant. A froth provides a rapid quenching efiectsimilar to water due to its relatively high specific heat, heatconductivity and heat of vaporization but without the damaging inertialefl'ect due to the liquid mass. The method is therefore ideally suitedto the manufacture of fibers and filaments of metals having a highlatent heat of fusion and/or a high surface tension.

It is, therefore, an object of this invention to provide a new andimproved method for the direct production of filaments and fibers ofappreciable length from metals having a relatively high latent heat offusion and/or a relatively high surface tension.

It is another object of this invention to provide a method for thedirect production of metallic filaments and fibers by extruding a moltenmetal stream into a froth quenchant.

BRIEF DESCRIPTION OF THE DRAWING The attached FIGURE illustratedapparatus, shown in partial cross-section, as may be used in thepractice of this invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT The embodiment of the apparatusshown in the drawing essentially comprises means for injecting a moltenmetal stream into a vertically disposed froth column over a frothingsolution such that the extruded molten filament will quickly solidify asit falls through the froth column prior to settling in the frothingsolution. More specifically, the apparatus shown comprises an open tank10 containing a frothing solution 12. The frothing solution 12 may be anaqueous soap solution or any similar solution, aqueous or nonaqueous,having a suitable surface tension as will form a reasonably stable frothhaving relatively high specific heat, heat conductivity and heat ofvaporization values.

An elongated froth stand pipe 14, open at both ends, or any walled bodydefining a laterally confined space, is vertically disposed over thefrothing solution 12 with the lower open end thereof immersed below thesurface of solution 12. Any means, such as flange supports 16 securedradially to the outer walls of stand pipe 14, may be used to maintainstand pipe 14 in a vertical position over the frothing solution 12. Atleast one conduit 18 is provided through or under the wall of stand pipe14 at a point below the surface of solution 12, to admit compressed airor gas thereto and thereby form a froth 20 which will raise in standpipe 14 above the frothing solution 12.

The extrusion equipment, such as a thermally insulated vessel 22 havingan extrusion nozzle 24, is positioned above the upper open end of standpipe 14 with the extrusion nozzle 24 aiming downward at approximatelythe axis of stand pipe 14. Vessel 22 should be thermally insulated andmay also be provided with a suitable heating means such as an inductioncoil (not shown) to maintain the metal therein in molten form.

To produce metallic filaments with the above described apparatus, tank10 must be filled with a frothing solution 12, as described, to a levelthat will submerge the lower end of stand pipe 14 and the outlet ofconduit 18. Compressed air, or other suitable gas, is admitted throughconduit 18 at a rate sufficient to maintain an upwardly moving column offroth 20 within stand pipe 14. The filament forming metal, in moltenstate, is extruded from vessel 22 through nozzle 24 by any conventionalextrusion technique. The extruded molten metal stream or streams,indicated by lines 26, falls downward due to gravitational pull, andmomentum countercurrently through the upward moving froth column 20where they quickly solidify during the fall due to direct contact withthe froth 20. The filaments are then collected at the bottom of tank 10,in solution 12, and are thereby further cooled to the solutiontemperature.

The froth column 20, being a mixture of liquid and gas, will cool themolten metal stream much more quickly than will pure gaseous quenchantssince thin films of liquid, i.e., bubble surfaces, are repeatedlydeposited on the stream surface as the stream falls therethrough. Theliquid film, of course, provides a greater cooling effect than wouldpure gas because of its heat of vaporization and higher specific heatand heat conductivity. It is apparent that the amount of liquid involvedin heat transfer is not great and would perhaps be quite insignificantin quenching hot metal pieces of substantial volume. In thisapplication, however, the molten metal stream is quite thin, providing asubstantial surface area to volume ratio, so that the small amount ofliquid film quenchant involved is quite effective in quickly quenchingthe stream' without damaging inertial effects.

As indicated above, this invention is particularly directed towards theuse of those metals which have a high latent heat of fusion and/or ahigh surface tension, so that the metal can be quickly solidified beforeit is pinched off at the nozzle in extremely short fibers. To this end,therefore, the spacing between the froth 20 and nozzle 24 should be asclose as possible without actual contact. It is most essential that thefroth should not impinge on the nozzle as direct contact will eithercause the nozzle orifice to be plugged due to premature metalsolidification or cause the nozzle to crack due to thermal shock. Forthis reason, it is preferable to space the nozzle 20 somewhat above theterminus of stand pipe 14, and then controlling the air flow ratethrough conduit 18 to assure that the froth 20 crests as close aspossible to the nozzle without contact therewith. Nozzle to frothspacings of 2 to 3 inches are usually satisfactory.

The distance of travel of the molten stream through the froth column 20must be sufiicient to ensure that the stream is solidified before itfalls into the solution 12. This will vary depending upon streamdiameter, velocity and temperature, as well as the rate at which thefroth is rising. As a practical matter, a 48-inch froth column provedsatisfactory for most applications.

For most applications, control and maximization of froth formation rateis most essential for optimum production. Whenever suitable fibers arenot being formed, an increase in froth fonnation rate will frequentlysolve the problem. For example, if the punch off" distance is decreasingdue to a decreasing nozzle orifice diameter, an increase in frothformation rate will shorten the spacing between nozzle and froth tocompensate therefor. On the other hand, when the metal is notsolidifying fast enough due to thicker filaments caused by an increasingnozzle orifice diameter, an increase in froth formation rate may benecessary to remove the greater heat of fusion. An increase in metalstream velocity may necessitate an increase in froth formation rate toremove the heat faster.

To give one specific example of the above process in more detail, castiron filaments having a diameter of 0.010 inch have been produced withapparatus substantially as shown utilizing a simple soap solution andcompressed air to produce bubbles approximately three-fourths inch indiameter. The stand pipe was 3 inches in diameter and provided a frothcolumn apfiff'fififlilil2hifl$ffi3 2*fll'33f2lll froth movement wasvaried from 0.2 to 0.8 gallons of water per minute, and the molten ironwas extruded at 2,240F and at a rate of approximately 20 ft./sec. Theresulting 0.010 inch iron filaments measured from 56 to 4 inches inlength.

Although the above described embodiment is rather specific, it should beapparent that other embodiments and modifications could readily beutilized without departing from the basic concept of the invention. Forexample, the invention is said to be primarily directed to theproduction of filaments from metals having high surface tensions or highlatent heats of fusion. Although this process does solve particularproblems in processing such metals, many other metals not falling intothis category could as easily be handled by this process. Although theapparatus and process described above is ideal for single streamlaboratory production, commercial operations could easily incorporatemany advantageous modifications, such as extruding a plurality ofstreams simultaneously. A larger solution tank 10 could then be utilizedto eliminate the need for stand pipe 14, the walls of the larger tankserving to confine the froth as a large mass. In addition, a moreintricate built-in frothing system could be devised to replace thesingle conduit 18, and a wide variety of frothing solutions, bothaqueous and organic, could be used.

Iclaim:

l. The method of producing metallic filaments and fibers from moltenmetal comprising extruding a stream of molten metal directly into afroth mass of predetermined configuration such that the metal streamwill solidify into a metallic filament while falling through said frothmass.

2. The method of claim 1 in which said froth is continuously formed overa frothing solution by bubbling compressed gas into said solution.

3. The method of claim 2 in which said continuously formed froth iscaused to rise within a stand pipe counter to the molten metal filamentsfalling therethrough.

2. The method of claim 1 in which said froth is continuously formed over a frothing solution by bubbling compressed gas into said solution.
 3. The method of claim 2 in which said continuously formed froth is caused to rise within a stand pipe counter to the molten metal filaments falling therethrough. 